(1) Field of the Invention
The present invention relates to a method for producing and using oak in divided form for flavoring wine. In particular, the present invention relates to a method wherein the oak in the divided form is pretreated to eliminate undesirable flavors and toasted and is then used in the wine over a relatively long period of time to develop the flavor.
(2) Prior Art
White oak is traditionally used to make barrels for aging wine. European white oak (Q. robur) is preferred for the flavor which it imparts to the wine. American white oak (Q. alba) is judged to be inferior for this purpose. Stainless steel or glass lined tanks have advantages, including lower cost, better protection against oxygen, better space utilization in the cellar and smaller losses from evaporation. The problem is that the tank wines do not taste like the oak barrel-aged wines. Oak barrels are thus often preferred.
Oak possesses xylem rays, and these dense rays of parenchyma tissue serve to make a stave relatively impermeable to liquids when the staves are quarter sawn or split. This causes the interface between liquid and wood to be perpendicular to the direction of the rays. A second property of oak that makes it attractive for storing liquids is the presence of tyloses in the xylem tissue. Tyloses are gum materials which plug the vessels, keeping them impermeable to wine or spirits. Wood that does not contain tyloses and xylem rays is unacceptable for cooperage. The third property of oak that makes it attractive for wine and spirit storage is the array of flavors imparted to the wine or spirits stored in it. The flavor components of oak are derived from phenolic acids, aromatic aldehydes, and possibly from small amounts of lactones and furfurals. Woods that possess the necessary physical attributes for tight cooperage such as beech, redwood, and cedar have been tried but none have met with the success of oak, primarily because of these flavor contributions, but could be used if the required flavor could be introduced.
When wines are aged in oak, they extract the flavoring constituents from the barrel. A second phenomenon that occurs during barrel aging is the evaporation of water and ethanol through the barrel staves. Depending on humidity conditions in the cellar, water may be lost to a greater or lesser extent than ethanol. The balance point between the two is 60 to 65% relative humidity. Above this, ethanol is lost at a greater rate and will decrease in the wine. Below this, water is lost at a greater rate and will cause the ethanol content to increase. The net loss of ethanol and water tends to concentrate the higher molecular weight volatiles since these are not lost as readily through the staves. Evaporative losses through a barrel are approximately 2 to 5% of the volume per year.
A third aspect of barrel aging of wine is the oxidation of the wine that occurs in the barrel. Aging of wine involves a slow oxidative change of the many constituents present. Barrel aging is unique in that it allows for introduction of oxygen into the wine. Many people harbor the erroneous belief that oxygen penetrates the staves and allows for further oxidation of the wine. However, Peterson (Peterson, R. G., Formation of reduced pressure in barrels during aging. Am. J. Enol. Vitic. 27:80-81 (1976)) demonstrated that as water and ethanol evaporate from the barrel, ullage (or a vacuum) develops. If oxygen were penetrating the barrel it would relieve this vacuum. It is more likely that oxygen is introduced when the barrels are topped or racked since these are standard practices for barrel aged wines.
The flavoring constituents extracted from oak are composed principally of non-flavonoid phenolic acids (gallic, gentisic, protocatechuic, p-hydroxybenzoic, p-coumaric, ferulic, sinapic and vanillic) and aromatic aldehydes (syringaldehyde, sinapaldehyde, vanillin, coniferaldehyde, propoiovanillone, acetosyringone) and smaller amounts of lactones and furfurals (Litchev, V. Am. J. Enol. Vitic. 40(1)31-35, (1989); Puech, J. L., Am. J. Enol. Vitic. 32(2) 111-14 (1981); and Puech, J. L., Am. J. Enol. Vitic. 38(3) 236-38 (1987). The relative quantities of the various components vary with the species of oak, the site on which the trees of a particular species were grown and differences within the tree itself. The species effect is under genetic control and is responsible for the largest differences found between one type of oak and another with the European white oak (Q. robur and Q. sessilis) containing larger amounts of phenolic compounds, than the American white oak (Q. alba). However, the site at which a tree is grown can exert subtle influences on the flavor makeup of the wood since it is the mesoclimate of the site that determines the relative amounts of spring and summer wood. Since spring wood is more porous than summer wood, trees growing on cooler sites have less dense wood since the ratio of spring to summer wood is high. Conversely, trees grown in warmer regions have wood of higher density since the ratio of spring to summer wood is high. This is a great simplification since things like rainfall and soil also play a role but it is generally true (Singleton, V. L., Some aspects of the wooden container as a factor in wine maturation). Less dense wood then, would allow a wine to penetrate the staves and extract the flavor components more quickly.
Once wine is placed in contact with the wood, the phenols are readily extracted. Rous (Rous, C. and B. Alderson. Am. J. Enol. Vitic. 34(4) 211-15 (1983)) showed that first and second fill barrels of both Q. alba and Q. robur yielded phenols to the wine by diffusion. However, after the most easily extracted phenols were removed, the kinetics of extraction were more linear, suggesting hydrolysis kinetics with bound phenols making up the bulk of those extracted. Each subsequent fill of the barrels showed slower phenol uptake than the previous fill. This is intuitively what one would expect and explains quantitatively why each successive barrel filling must remain longer to get the desired effect on wine quality.
While the species effect on phenol and aromatic aldehyde content is in many cases significant, various researchers have disagreed as to the implications of this on wine quality. Aiken and Noble (Aiken, J. W., et al., Am. J. Enol. Vitic. 35(4) 196-99 (1984); and Aiken, J. W., et al., Vitis 23:27-36 (1984)) aged Cabernet Sauvignon in American and French Oak barrels coopered by the same methods. A panel of tasters were able to detect a difference between the wines stored in the two barrel types after 115 days of aging. However, the same panel was not able to detect differences between barrels when the wines were compared after 239 and 338 days of aging. Rous and Alderson (Rous, C., et al., Am. J. Enol. Vitic. 34(4) 211-15 (1983)) aged Sauvignon blanc and a white blend in French and American oak barrels and found significant differences between the wines aged in the different barrel types. Their barrels were also coopered by the same methods. The American oak was from Missouri and the French wood was from the Nevers Forest as in the previous study but the wines were aged for a maximum of 175 days. This raises some interesting questions. First, there is a question whether the difference between barrels is only noticeable in wines stored in barrels for relatively short periods of time, or are wood flavor differences more easily masked by red wine. Second, the barrels used in the two studies were coopered by different firms. Since toast levels are subjective, a light toast to one firm may be a medium or heavy toast to another. It is possible that cooperage technique has the most important influence on the properties of the barrel as suggested by Aiken and Noble. This seems unlikely since other work has clearly shown the differences in structure and chemistry of the different species (Puech, J. L., Am. J. Enol. Vitic. 32(2) 111-14 (1981); Rous, C. and B. Alderson., Am. J. Enol. Vitic. 34(4) 211-15 (1983); Singleton, V. L., et al., Am. J. Enol. Vitic. 22: 161-6 (1971); and Singleton, V. L., Some aspects of the wooden container as a factor in wine maturation. In: Chemistry of Winemaking (A. D. Webb ed.). Am. Chem. Soc., Washington, D. C., pp. 254-77 (1974)). However, Litchev (Litchev, V. Influence of Oxidation process on the development of the taste and flavor of wine distillates. Am. J Enol. Vitic. 40 (1)31-35 (1989)) showed that toasting of oak at 200.degree. C. increased the aromatic aldehyde content from 12 to 60 times depending on the compound being evaluated. They proposed that lignin was degraded to aromatics by toasting and that lignin was released from its complexed form by toasting and made more available for ethanolysis. Lignin ethanolysis produces a further increase in released aromatics. After the extractable flavor components of oak (i.e. phenolic acids and aromatic aldehydes) are in solution, they are subjected to conversion via oxidation and esterification to produce a wide range of compounds. These reactions occur over a long period of time (4 years in this study) which helped explain why oak flavors soften over time in the bottle. So, while species determines the absolute amount of various flavors available for extraction, cooperage technology influences the rate and portion of the total which is available for flavoring the wine.
If one were attempting to maximize the amount of vanillin in a wine, the best way to achieve this would be to use a heavily toasted barrel and allow the wine to stay in the barrel for as long as possible up to about a year since this would allow for the ethanolysis of lignin. The time frame used to allow ethanolysis was 6 months in the Litchev work (Litchev, V., Am. J. Enol. Vitic. 40(1)31-35, (1989)). Researchers have also shown the products of ethanolysis increase over a long period of time (Puech, J. L., Am. J. Enol. Vitic. 32(2) 111-14 (1981); and Puech, J. L., Am. J. Enol. Vitic. 38(3) 236-38 (1987)). After longer aging periods, the products of ethanolysis may play an increasing role in the wines flavor and may serve to obscure the differences in the phenolics. The differences are reduced further over time as oxidation and esterification change the components in the wine. Further, after the first couple of uses of the barrel, the easily extracted phenols are removed so the proportion of lignin ethanolysis products to phenols may be greater in wines from third-use barrels and differences more difficult to detect.
Since oak cooperage is expensive to purchase and maintain, only the wines that have the greatest potential are typically aged in barrels. It has been suggested that oak chips could be used to introduce some of the desirable oak flavors into wines of lesser value (Singleton, V. L., et al., Am. J. Enol. Vitic. 12: 152-8 (1961); Singleton, V. L., et al., Am. J. Enol. Vitic. 22: 161-6 (1971); and Wilker, K. L. and J. F. Gallander., Am. J. Enol. Vitic. 39(1) 38-43 (1988)). This has met with some success but has its limitations. Extracts made from chips are of new chips which introduces large amount of the phenols. Since the chips are typically extracted for relatively short periods of time (4 to 7 days) there is little time for lignin ethanolysis. Consequently there is a large ratio of phenols to aromatic aldehydes. Further, the chips are rarely toasted thereby reducing the aromatic aldehydes available. Finally, using oak chips to flavor a wine does not introduce oxygen into the wine or allow for the concentration of high molecular weight volatiles as would be the case when barrel aging. Using oak chips in barrels that no longer impart flavors to wine would be one way around this and it also allows for the continued use of old barrels (Wilker, K. L., et al., Am. J. Enol. Vitic. 39(1)38-43 (1988)). However, there still remains the question of how the chips should be handled.
The current prescribed method of using oak chips calls for: (1) a 2-day extraction of the chips with wine to produce a concentrated extract; 2) then add a portion of the extract to a large batch of wine. In a recent study, Wilker and Gallander, Am. J. Enol Vitic. 39(1) 38-43 (1988) reported results of a 14-day addition of raw chips into a used wine barrel compared to wine stored in a new barrel. The results were a difference in taste of the wines over oak barrels. The introduction of chips for a short period of time into the wine has also been suggested. In this instance, the vanillins and precursors are not taken from the chips since this requires ethanolysis of the lignin which takes several months.
To date, those working with oak chips have suggested making ethanol and water extracts of said chips and then adding the resulting extract to the wine in order to give the wine "artificial barrel aging". The amount of extract added was determined by conducting laboratory trials in which increasing amounts of extract were added to the wine until the desired flavor was reached. Several difficulties have been found with this approach. First, by making an extract over the course of only a few days, one produces an extract containing many of the undesirable flavors associated with oak and only a portion of those flavors which are desired. This combination is then added to the wine.
A way was needed to introduce the divided wood into the wine so it could remain there for extended periods but not interfere with winery operations. This would give wineries an inexpensive means for adding the complexity in flavor associated with barrel aging without incurring the huge cost of purchasing new cooperage. Currently, barrels constructed using French-grown oak sell for $525 per 223 liter barrel. These barrels, on the average, will impart oak flavor to wines for about six years with the amount of flavor greatly diminished the last two years.